Sputtering target, oxide semiconductor thin film, and method for producing these

ABSTRACT

A sputtering target including an oxide that includes an indium (In) element, a tin (Sn) element, a zinc (Zn) element and an aluminum (Al) element, wherein the oxide includes a homologous structure compound represented by InAlO 3 (ZnO) n , (m is 0.1 to 10) and a bixbyite structure compound represented by In 2 O 3 .

TECHNICAL FIELD

The invention relates to a sputtering target, a thin film prepared byusing the target and a thin film transistor that comprises the thinfilm.

BACKGROUND ART

Field effect transistors, such as a thin film transistor (TFT), arewidely used as the unit electronic device of a semiconductor memoryintegrated circuit, a high frequency signal amplification device, adevice for driving a liquid crystal, or the like, and they areelectronic devices which are currently most widely put into practicaluse. Among these, with significant development in displays in recentyears, in various displays such as a liquid crystal display (LCD), anelectroluminescence display (EL) and a field emission display (FED), aTFT is frequently used as a switching device which drives a display byapplying a driving voltage to a display device.

As a material of a semiconductor layer (channel layer) which is a mainelement of a field effect transistor, a silicon semiconductor compoundis used most widely. Generally, a silicon single crystal is used for ahigh frequency amplification device, a device for integrated circuits orthe like which need high-speed operation. On the other hand, anamorphous silicon semiconductor (amorphous silicon) is used for a devicefor driving a liquid crystal or the like in order to satisfy the demandfor realizing a large-area display.

A thin film of amorphous silicon can be formed at relatively lowtemperatures. However, the switching speed thereof is slow as comparedwith that of a crystalline thin film. Therefore, when it is used as aswitching device that drives a display, it may be unable to follow thedisplay of a high-speed animation. Specifically, amorphous siliconhaving a mobility of 0.5 to 1 cm²Ns could be used in a liquid crystaltelevision of which the resolution is VGA. However, if the resolution isequal to or more than SXGA, UXGA and QXGA, a mobility of 2 cm²Ns or moreis required. Moreover, if the driving frequency is increased in order toimprove the image quality, a further higher mobility is required.

On the other hand, as for a crystalline silicon-based thin film,although it has a high mobility, there are problems that a large amountof energy and a large number of steps are required for the production,and that large-area film formation is difficult. For example, when asilicon-based thin film is crystallized, a high temperature of 800° C.or more or laser annealing which needs expensive equipment is required.In the case of a crystalline silicon-based thin film, the deviceconfiguration of a TFT is normally restricted to a top-gateconfiguration, and hence, reduction in production cost such as decreasein number of masks is difficult.

In order to solve the problem, a thin film transistor using an oxidesemiconductor film formed of indium oxide, zinc oxide and gallium oxidehas been studied. In general, an oxide semiconductor thin film is formedby sputtering using a target (sputtering target) composed of an oxidesintered body.

For example, a target formed of a compound showing a homologous crystalstructure such as that represented by a general formulas In₂Ga₂ZnO₇ andInGaZnO₄ is known (Patent Documents 1, 2 and 3). However, in thistarget, in order to increase the sintering density (relative density),it is required to conduct sintering in an oxidizing atmosphere. In thiscase, in order to reduce the resistance of the target, a reductiontreatment at a high temperature is required to be conducted aftersintering. Further, if the target is used for a long period of time,problems arise that the properties of the resulting film or thefilm-forming speed largely change; abnormal discharge due to abnormalgrowth of InGaZnO₂ or In₂Ga₂ZnO₇ occurs; particles are frequentlygenerated during film formation or the like. If abnormal dischargeoccurs frequently, plasma discharge state becomes unstable, and as aresult, stable film-formation is not conducted, adversely affecting thefilm properties.

On the other hand, a thin film transistor that is obtained by using anamorphous oxide semiconductor film that does not contain gallium and iscomposed of indium oxide and zinc oxide has been proposed (PatentDocument 4). However, this thin film transistor has a problem that anormally-off operation of a TFT cannot be realized if the oxygen partialpressure at the time of film formation is not increased.

Further, studies have been made on a sputtering target for forming aprotective layer of an optical information recording medium, that isobtained by adding an additive element such as Ta, Y, Si or the like toan In₂O₃—SnO₂—ZnO-based oxide composed mainly of tin oxide (PatentDocuments 5 and 6). However, these targets are not used for forming anoxide semiconductor and they have problems that an agglomerate of aninsulating material is likely to be formed easily, whereby theresistance is increased or abnormal discharge tends to occur easily.

RELATED ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-H08-245220

Patent Document 2: JP-A-2007-73312

Patent Document 3: WO2009/084537

Patent Document 4: WO2005/088726

Patent Document 5: WO2005/078152

Patent Document 6: WO2005/078153

SUMMARY OF THE INVENTION

An object of the invention is to provide a high-density andlow-resistant sputtering target.

Another object of the invention is to provide a thin film transistorhaving a high field effect mobility and high reliability.

In order to attain the above-mentioned object, the inventors of theinvention made extensive studies. As a result, the inventors have foundthat a sputtering target comprising an oxide that contains an indium(In) element, a tin (Sn) element, a zinc (Zn) element and an aluminum(Al) element and comprising a homologous structure compound representedby InAlO₃(ZnO)_(m) (m is 0.1 to 10) and a bixbyite structure representedby In₂O₃ has a high relative density and has a low resistance. Further,the inventors have found that a TFT using a thin film obtained by usingthis target in a channel layer has a high field effect mobility and hasa high reliability. The invention has been completed based on thesefindings.

According to the invention, the following sputtering target or the likeare provided.

1. A sputtering target comprising an oxide that comprises an indium (In)element, a tin (Sn) element, a zinc (Zn) element and an aluminum (Al)element, wherein the oxide comprises a homologous structure compoundrepresented by InAlO₃(ZnO)_(m) (m is 0.1 to 10) and a bixbyite structurecompound represented by In₂O₃.2. The sputtering target according to 1, wherein the homologousstructure compound is one or more selected from homologous structurecompounds represented by InAlZn₄O₇, InAlZn₃O₆, InAlZn₂O₅ and InAlZnO₂.3. The sputtering target according to 1 or 2, wherein the atomic ratioof In, Sn, Zn and Al satisfies the following formulas (1) to (4):

0.10≦In/(In+Sn+Zn+Al)≦0.75  (1)

0.01≦Sn/(In+Sn+Zn+Al)≦0.30  (2)

0.10≦Zn/(In+Sn+Zn+Al)≦0.70  (3)

0.01≦Al/(In+Sn+Zn+Al)≦0.40  (4)

wherein in the formulas In, Sn, Zn and Al independently indicate anatomic ratio of each element in the sputtering target.4. The sputtering target according to any one of 1 to 3 that has arelative density of 98% or more.5. The sputtering target according to any one of 1 to 4 that has a bulkspecific resistance of 10 mΩcm or less.6. The sputtering target according to any one of 1 to 5 that does notcomprise a spinel structure compound represented by Zn₂SnO₄.7. An oxide semiconductor thin film formed by a sputtering method withthe use of the sputtering target according to any one of 1 to 6.8. A method for producing an oxide semiconductor thin film, wherein thefilm is formed by a sputtering method with the use of the sputteringtarget according to any one of 1 to 6 in an atmosphere of a mixed gasthat comprises: one or more selected from water vapor, an oxygen gas anda nitrous oxide gas; and a rare gas.9. The method for producing an oxide semiconductor thin film accordingto 8, wherein the formation of the oxide semiconductor thin film isconducted in an atmosphere of a mixed gas that comprises a rare gas andat least water vapor.10. The method for producing an oxide semiconductor thin film accordingto 9, wherein the ratio of the water vapor contained in the mixed gas is0.1% to 25% in terms of a partial pressure ratio.11. The method for producing the oxide semiconductor thin film accordingto any one of 8 to 10 comprising:

transporting substrates in sequence to positions opposing to 3 or moreof the sputtering targets arranged in parallel with a prescribedinterval in a vacuum chamber;

applying a negative potential and a positive potential alternately froman AC power source to each of the targets; and

causing plasma to be generated on the target by applying at least oneoutput from the AC power source while switching the target to which apotential is applied among two or more targets that are divergentlyconnected to this AC power source, thereby forming a film on a substratesurface.

12. The method for producing an oxide semiconductor thin film accordingto 11, wherein the AC power density of the AC power source is 3 W/cm² ormore and 20 W/cm² or less.13. The method for producing an oxide semiconductor thin film accordingto 11 or 12, wherein the frequency of the AC power source is 10 kHz to 1MHz.14. A thin film transistor comprising, as a channel layer, the oxidesemiconductor thin film formed by the method for producing an oxidesemiconductor thin film according to any one of 8 to 13.15. The thin film transistor according to 14 that has a field effectmobility of 10 cm²/Vs or more.

According to the invention, it is possible to provide a high-density andlow-resistant sputtering target.

According to the invention, it is possible to provide a thin filmtransistor having a high field effect mobility and high reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a sputtering apparatus used in one embodimentof the invention;

FIG. 2 is a view showing an X-ray diffraction chart of a sintered bodyobtained in Example 1;

FIG. 3 is a view showing an X-ray diffraction chart of a sintered bodyobtained in Example 2;

FIG. 4 is a view showing an X-ray diffraction chart of a sintered bodyobtained in Example 3;

FIG. 5 is a view showing an X-ray diffraction chart of a sintered bodyobtained in Example 4;

FIG. 6 is a view showing an X-ray diffraction chart of a sintered bodyobtained in Example 5; and

FIG. 7 is a view showing an X-ray diffraction chart of a sintered bodyobtained in Example 6.

MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, a detailed explanation will be made on the sputteringtarget or the like of the invention. The invention is, however, notlimited to the following embodiment and examples.

1. Sputtering Target

The sputtering target of the invention comprises an oxide that containsan indium (In) element, a tin (Sn) element, a zinc (Zn) element and analuminum (Al) element, and is characterized in that it comprises ahomologous structure compound represented by InAlO₃(ZnO)_(m) (m is 0.1to 10) and a bixbyite structure represented by In₂O₃.

In the invention, due to the presence of the homologous structurecompound and the bixbyite structure compound mentioned above, the targethas a high relative density and a low bulk resistance. Therefore,occurrence of abnormal discharge can be suppressed when sputtering isconducted using the target of the invention. Further, the target of theinvention can form a high-quality oxide semiconductor thin filmefficiently, at a low cost and with a small energy.

The homologous structure compound represented by InAlO₃(ZnO)_(m) (m is0.1 to 10) can be confirmed by the fact that a peak derived from thehomologous structure compound can be observed as a result of an X-raydiffraction analysis of the target (oxide sintered body).

In the homologous structure compound represented by InAlO₃(ZnO)_(m) (mis 0.1 to 10), m is 0.1 to 10, preferably 0.5 to 8, and furtherpreferably 1 to 7. m is preferably an integer.

The homologous structure compound represented by InAlO₃(ZnO)_(m) (m is0.1 to 10) is preferably one or more selected from homologous structurecompounds represented by InAlZn₄O₇, InAlZn₃O₆, InAlZn₂O₅ and InAlZnO₂.

The homologous crystal structure is a crystal structure formed of along-period “natural superlattice” structure in which crystal layers ofdifferent substances are stacked. If the crystal period or the thicknessof each thin film layer is on an about nanometer level, a homologousstructure compound can exhibit inherent characteristics that differ fromthe characteristics of a single substance or a mixed crystal in whichthe layers are uniformly mixed depending on combination of the chemicalcomposition or the thickness of each layer.

The crystal structure of the homologous phase can be confirmed by a factthat an X-ray diffraction pattern measured from powder obtained bypulverizing the target coincident with an X-ray diffraction pattern ofthe crystal structure of the homologous phase assumed from thecomposition ratio. Specifically, it can be confirmed that the pattern iscoincident with the crystal structure X-ray diffraction pattern of thehomologous phase obtained from the JCPDS (Joint Committee of PowderDiffraction Standards) card or the ICSD (The Inorganic Crystal StructureDatabase).

As the oxide crystal having a homologous crystal structure, an oxidecrystal represented by RAO₃(MO)_(m) can be mentioned. R and A are each apositive trivalent metal element, and as examples thereof, In, Ga, Al,Fe, B or the like can be mentioned, for example. A is a positivetrivalent metal element that is different from R, and as examplesthereof, Ga, Al, Fe or the like can be mentioned, for example. M is apositive divalent metal element, and as examples thereof, Zn, Mg or thelike can be mentioned, for example. In the homologous structure compoundrepresented by InAlO₃(ZnO)_(m) (m is 0.1 to 10) of the invention, R isIn, A is Al and M is Zn.

The homologous structure represented by InAlZnO₂ shows a peak pattern ofNo. 40-0258 of JCPDS database or a similar (shifted) pattern.

The homologous structure represented by InAlZn₂O₅ shows a peak patternof No. 40-0259 of JCPDS database or a similar (shifted) pattern.

The homologous structure represented by InAlZn₃O₆ shows a peak patternof No. 40-0260 of JCPDS database or a similar (shifted) pattern.

The homologous structure represented by InAlZn₄O₇ shows a peak patternof 40-0261 of JCPDS database or a similar (shifted) pattern.

The bixbyite structure compound represented by In₂O₃ can be confirmed bya fact that the peak derived from the bixbyite structure compound isobserved by an X-ray diffraction analysis of the target.

The bixbyite structure compound represented by In₂O₃ shows a peakpattern of No. 06-0416 of JCPDS database or a similar (shifted) pattern.

The bixbyite is also referred to as a C-type rare-earth oxide or Mn₂O₃(I) type oxide. As stated in the “Technology of Transparent ConductiveFilm” (published by Ohmsha Ltd., edited by Japan Society for thePromotion of Science, transparent oxide/photoelectron material 166committee, 1999) or the like, this compound has a chemicalstoichiometric ratio of M₂X₃ (M is a cation and X is an anion, which isnormally an oxygen ion), and one unit cell is formed of 16 M₂X₃molecules and 80 atoms in total (the number of M is 32 and the number ofX is 48).

The bixbyite structure compound includes a substitutional solid solutionin which part of atoms or ions in a crystal structure are replaced byother atoms and an interstitial solid solution in which other atoms areadded to a position between lattices.

The sputtering target of the invention preferably does not comprise aspinel structure compound represented by Zn₂SnO₄.

The spinel structure compound represented by Zn₂SnO₄ shows a peakpattern of No. 24-1470 of JCPDS database or a similar (shifted) patternin an X-ray diffraction.

As stated in “Crystal Chemistry” (Mitsuoki Nakahira, Kodansha Ltd.,1973) or the like, the spinel structure normally means AB₂X₄ type orA₂BX₄ type structure, and a compound having such a crystal structure isreferred to as a spinel structure compound.

In general, in a spinel structure, cations (normally, oxygen) are cubicclose-packed, and anions are present in part of the tetrahedralinterstitial site or the octahedral interstitial site thereof. Asubstitutional solid solution in which a part of atoms or ions in acrystal structure are replaced by other atoms and an interstitial solidsolution in which other atoms are added to a position between latticesare included in a spinel structure compound.

If the target comprises a spinel structure compound represented byZn₂SnO₄ in addition to the homologous structure compound represented byInAlO₃(ZnO)_(m) (m is 0.1 to 10) and a bixbyite structure compoundrepresented by In₂O₃, the sputtering speed varies depending on thecrystal phase since a large amount of different phases are present inthe oxide sintered body constituting the target, and hence, partsremaining unremoved may be formed.

In the target of the invention, it is preferred that an indium element,a tin element, a zinc element and an aluminum element satisfy thefollowing formulas (1) to (4):

0.10≦In/(In+Sn+Zn+Al)≦0.75  (1)

0.01≦Sn/(In+Sn+Zn+Al)≦0.30  (2)

0.10≦Zn/(In+Sn+Zn+Al)≦0.70  (3)

0.01≦Al/(In+Sn+Zn+Al)≦0.40  (4)

wherein in the formula, In, Sn, Zn and Al independently indicate theatomic ratio of each element in the sputtering target.

In the formula (1), if the atomic ratio of In is 0.10 or more, the bulkresistance of the sputtering target does not increase too high, wherebyDC sputtering can be conducted without problems. On the other hand, ifthe atomic ratio of In is 0.75 or less, the homologous structurecompound represented by InAlO₃(ZnO)_(m) (m is 0.1 to 10) and thebixbyite structure compound represented by In₂O₃ tend to be formedeasily in the target. If both of the homologous structure compoundrepresented by InAlO₃(ZnO)_(m) (m is 0.1 to 10) and the bixbyitestructure compound represented by In₂O₃ are formed, abnormal graingrowth of crystals in the oxide and abnormal discharge caused by theabnormal grain growth can be prevented.

From the above, it is preferred that the atomic ratio of the In element[In/(In+Sn+Zn+Al)] be 0.10 to 0.75, more preferably 0.20 to 0.70, andfurther preferably 0.25 to 0.70.

In the formula (2) above, if the atomic ratio of the Sn element is 0.01or more, the density of the oxide sintered body that constitutes thetarget sufficiently increases, whereby the bulk resistance of the targetis lowered. On the other hand, if the atomic ratio of the Sn element is0.30 or less, SnO₂ that causes abnormal discharge to occur issuppressed, whereby abnormal discharge can be prevented.

From the above, the atomic ratio of the Sn element [Sn/(In+Sn+Zn+Al)] ispreferably 0.01 to 0.30, more preferably 0.03 to 0.25, and furtherpreferably 0.05 to 0.15.

In the above formula (3), if the atomic ratio of the Zn element is 0.10or more, the homologous structure compound represented byInAlO₃(ZnO)_(m) (m is 0.1 to 10) tends to be formed easily. On the otherhand, if the atomic ratio of the Zn element is 0.70 or less, ZnO is notdeposited easily, and as a result, abnormal discharge caused by ZnO canbe prevented.

From the above, the atomic ratio of the Zn element [Zn/(In+Sn+Zn+Al)] ispreferably 0.10 to 0.70, more preferably 0.20 to 0.65, and furtherpreferably 0.25 to 0.60.

In the above formula (4), if the atomic ratio of the Al element is 0.01or more, the homologous structure compound represented byInAlO₃(ZnO)_(m) (m is 0.1 to 10) tends to be formed easily. On the otherhand, if the atomic ratio of the Al element is 0.40 or less, formationof Al₂O₃ that causes abnormal discharge to occur is suppressed, wherebyoccurrence of abnormal discharge can be prevented.

From the above, the atomic ratio of the Al element [AI/(In+Sn+Zn+Al)] ispreferably 0.01 to 0.40, more preferably 0.02 to 0.30, and furtherpreferably 0.05 to 0.25.

The atomic ratio of elements contained in the sintered body can beobtained by quantitatively analyzing the elements contained withInduction Coupled Plasma Atomic Emission Spectrometry (ICP-AES).

Specifically, when a solution sample is nebulized using a nebulizer, andintroduced into an argon plasma (about 6000 to about 8000° C.), eachelement contained in the sample absorbs thermal energy, and is excited,and the orbital electrons migrate from the ground state to the orbitalat a high energy level. The orbital electrons then migrate to theorbital at a lower energy level when about 10⁻⁷ to about 10⁻⁸ secondshave elapsed. In this case, the difference in energy is emitted aslight. Since the emitted light has an element-specific wavelength(spectral line), the presence or absence of each element can bedetermined based on the presence or absence of the spectral line(qualitative analysis).

Since the size (luminous intensity) of each spectral line is inproportion to the number of each element contained in the sample, theelement concentration in the sample can be determined by comparison witha standard solution having a known concentration (quantitativeanalysis).

After specifying the elements contained in the sample by qualitativeanalysis, the content of each element is determined by quantitativeanalysis, and the atomic ratio of each element is calculated from theresults.

The sputtering target of the invention may comprise other metal elementsthan In, Sn, Zn and Al as long as the effects of the invention are notimpaired. The sputtering target may consist essentially of In, Sn, Znand Al or may consist of In, Sn, Zn and Al.

In the invention, the “essentially” means that 95 mass % or more and 100mass % or less (preferably 98 mass % or more and 100 mass % or less) ofthe metal element of the sputtering target is In, Sn, Zn and Al. Thesputtering target of the invention may contain impurities that areinevitably mixed in addition to In, Sn, Zn and Al in a range that doesnot impair the advantageous effects of the invention.

It is preferred that the sputtering target of the invention have arelative density of 98% or more. Particularly, if an oxide semiconductoris deposited on a large-sized substrate (G1 or more) with an increasedsputtering output, it is preferred that the relative density be 98% ormore.

The relative density is a density which is relatively calculated for thetheoretical density which is calculated from the weighted average. Thedensity calculated from the weighted average of the density of each rawmaterial is a theoretical density, which is assumed to be 100%.

If the relative density is 98% or more, stable sputtering state ismaintained. When the film is formed by increasing the sputtering poweron a large substrate, if the sputtering target has a relative density98% or more, the target surface is not be blackened easily or abnormaldischarge does not occur easily. The relative density is preferably98.5% or more, with 99% or more being more preferable.

The relative density of the target can be measured by the Archimedianmethod. The relative density is preferably 100% or less. If the relativedensity is 100% or less, metal particles are not be generated easily ina sintered body, and formation of a lower oxide is suppressed.Therefore, it is not required to control the oxygen supply amount duringfilm-formation strictly.

Further, the density can be adjusted by a post treatment or the likesuch as a heat treatment in the reductive atmosphere after sintering. Asthe reductive atmosphere, an atmosphere such as argon, nitrogen andhydrogen, or an atmosphere of a mixture of these gases can be used.

It is desired that the maximum particle size of the crystal in the oxideconstituting the sputtering target be 8 μm or less. Due to the crystalparticle size of 8 μm or less, formation of nodules can be suppressed.

When the target surface is ground by sputtering, the grinding speeddiffers depending on the direction of the crystal, whereby unevenness isgenerated on the target surface. The size of this unevenness variesdepending on the particle size of the crystal present in the sinteredbody. It is assumed that, in the target formed of a sintered body havinga large crystal particle size, a greater scale of unevenness occurs, andnodules are generated from this convex part.

The maximum particle size of the crystal of the sputtering target isobtained as follows. If the sputtering target has a circular shape, atfive locations in total, i.e. the central point (one) and the points(four) which are on two central lines crossing orthogonally at thiscentral point and are middle between the central point and theperipheral part, or if the sputtering target has a square shape, at fivelocations in total, i.e. the central point (one) and middle points(four) between the central point and the corners of the diagonal linesof the square, the maximum diameter is measured for the biggest particleobserved within a 100-μm square. The maximum particle size is theaverage value of the maximum diameters of the biggest particle presentin each of the frames defined by the five locations. As for the particlesize, the longer diameter of the crystal particle is measured. Thecrystal particles can be observed by the scanning electron microscopy(SEM).

In the sputtering target of the invention, the bulk specific resistanceis preferably 10 mΩcm or less, more preferably 8 mΩcm or less, with 5mΩcm or less being particularly preferable. The bulk specific resistancecan be measured by the method described in the Examples.

As for the relative density and the bulk resistance of the target, therelative density of the target can be 98% or more and the bulkresistance can be 10 mΩcm or less by adjusting the content of the indium(In) element, the tin (Sn) element, the zinc (Zn) element and thealuminum (Al) element to be in the above-mentioned ranges (1) to (4).

The method for producing a sputtering target of the invention comprisesthe following two steps, for example:

(1) A step in which raw material compounds are mixed and formed toobtain a formed body(2) A step in which the above-mentioned formed body is sintered

Hereinbelow, an explanation will be made based on these steps.

(1) A Step in which Raw Material Compounds are Mixed and Formed toObtain a Formed Body

No specific restrictions are imposed on the raw material compounds, andthe raw material compounds may be a simple substance or a compound ofIn, Sn, Zn and Al. It is preferable to use a simple substance or acompound such that the sintered body can have an atomic ratio shown inthe following formulas (1) to (4):

0.10≦In/(In+Sn+Zn+Al)≦0.75  (1)

0.01≦Sn/(In+Sn+Zn+Al)≦0.30  (2)

0.10≦Zn/(In+Sn+Zn+Al)≦0.70  (3)

0.01≦Al/(In+Sn+Zn+Al)≦0.40  (4)

wherein In, Sn, Zn and Al independently indicate the atomic ratio ofeach element in the sputtering target.

A combination of indium oxide, tin oxide, zinc oxide and aluminum metal,a combination of indium oxide, tin oxide, zinc oxide and aluminum oxideor the like can be mentioned. It is preferred that the raw material bepowder. It is preferred that the raw material be a mixed powder ofindium oxide, tin oxide, zinc oxide and aluminum oxide.

If a simple substance of a metal is used as a raw material; e.g. when acombination of indium oxide, tin oxide, zinc oxide and aluminum metal isused as raw material powders, metal particles of aluminum may be presentin the resulting sintered body. As a result, metal particles on thetarget surface may be molten during film formation and hence may not beemitted from the target, resulting in a great difference between thecomposition of the film and the composition of the sintered body.

The average particle diameter of the raw material powder is preferably0.1 μm to 1.2 μm, more preferably 0.1 μm to 1.0 μm. The average particlediameter of the raw material powder can be measured by a laserdiffraction particle size distribution measuring apparatus or the like.

For example, In₂O₃ powder having an average particle diameter of 0.1 μmto 1.2 μm, SnO₂ powder having an average particle diameter of 0.1 μm to1.2 μm, ZnO powder having an average particle diameter of 0.1 μm to 1.2μm and Al₂O₃ powder having an average particle diameter of 0.1 μm to 1.2μm are used as the raw material powder. They are mixed in an amountratio that satisfies the above-mentioned formulas (1) to (4).

In the case of the step (1), the method for forming is not particularlyrestricted, and a known method can be used. For example, a water-basedsolvent is added to raw material powders including indium oxide powder,tin oxide powder, zinc oxide powder and aluminum oxide powder, and theresulting slurry is mixed for 12 hours or more. Then, the mixture issubjected to solid-liquid separation, dried and granulated, and thegranulated product is then put in a mold and formed.

For the mixing, a wet or dry ball mill, a vibration mill, a beads millor the like can be used. In order to obtain uniform and fine crystalparticles and voids, the most preferable method is a beads mill mixingmethod since it can pulverize the aggregate efficiently for a shortperiod of time and can realize a favorable dispersed state of additives.

When a ball mill is used for mixing, the mixing time is preferably 15hours or longer, more preferably 19 hours or longer. If the mixing timeis 15 hours or longer, a high-resistant compound such as Al₂O₃ may notbe generated easily in the oxide sintered body finally obtained. When abeads mill is used for pulverizing and mixing, the mixing time variesdepending on the size of the apparatus used and the amount of slurry tobe treated. However, the mixing time is controlled appropriately suchthat the particle distribution in the slurry becomes uniform, i.e. allof the particles have a particle size of 1 μm or less.

It is preferred that an arbitrary amount of a binder be added, andmixing be conducted simultaneously with the addition of the binder. Asthe binder, polyvinyl alcohol, vinyl acetate or the like can be used.

Subsequently, granulated powder is obtained from a raw material powderslurry. For granulation, it is preferable to use quick dry granulation.As the apparatus for quick dry granulation, a spray dryer is widelyused. Specific drying conditions are determined according to conditionssuch as the concentration of slurry to be dried, the temperature of hotair used for drying and the amount of wind. For actually conducting thequick dry granulation, it is required to obtain optimum conditions inadvance.

In the case of quick dry granulation, a homogenous granulated powder canbe obtained. That is, separation of In₂O₃ powder, SnO₂ powder, ZnOpowder and Al₂O₃ powder due to difference in speed of sedimentationcaused by the difference in specific gravity of the raw material powdercan be prevented. If a sintered body is made by using uniform granulatedpowder, abnormal discharge during sputtering due to the presence ofAl₂O₃ or the like within the sintered body can be prevented.

The granulated powder can normally be formed into a formed body bypressing at a pressure of 1.2 ton/cm² or more, for example, by means ofa mold press or cold isostatic pressing (CIP).

(2) A Step in which the Formed Body is Sintered

The resulting formed body is sintered at 1200 to 1650° C. for 10 to 50hours to obtain a sintered body. The sintering temperature is preferably1350 to 1600° C., more preferably 1400 to 1600° C., and furtherpreferably 1450 to 1600° C. The sintering time is preferably 12 to 40hours, more preferably 13 to 30 hours.

If the sintering temperature is 1200° C. or more and the sintering timeis 10 hours or longer, formation of Al₂O₃ or the like within the targetcan be suppressed, whereby abnormal discharge can preferably beprevented. On the other hand, if the calcination temperature is 1650° C.or lower or the calcination time is 50 hours or shorter, an increase inaverage crystal diameter due to significant crystal particle growth canbe prevented. In addition, since generation of large voids can besuppressed, lowering in strength of a sintered body or occurrence ofabnormal discharge can preferably be prevented.

As the method of sintering used in the invention, in addition to thepressureless sintering, a pressure sintering method such as hotpressing, oxygen pressurization and hot isostatic pressing or the likecan be used. In respect of a decrease in production cost, possibility ofmass production and easiness in production of a large-sized sinteredbody, it is preferable to use pressureless sintering.

In the pressureless sintering, a formed body is sintered in the air orthe oxidizing gas atmosphere. Preferably, a formed body is sintered inthe oxidizing gas atmosphere. The oxidizing gas atmosphere is preferablyan oxygen gas atmosphere. It is preferred that the oxygen gas atmospherebe an atmosphere having an oxygen concentration of 10 to 100 vol %, forexample. In the method for producing the sintered body mentioned above,the density of the sintered body can be further increased by introducingan oxygen gas atmosphere during the temperature-elevating step.

As for the temperature-elevating rate at the time of sintering, it ispreferred that the temperature-elevating rate be 0.1 to 2.5° C./min in atemperature range of from 800° C. to a sintering temperature (1200 to1650° C.).

In the sputtering target of an oxide comprising an indium (In) element,a tin (Sn) element, a zinc (Zn) element and an aluminum (Al) element, atemperature range of from 800° C. and higher is a range where sinteringproceeds most quickly. If the temperature-elevating rate in thistemperature range is 0.1° C./min or more, excessive growth of crystalparticles can be prevented, whereby an increase in density can beattained. On the other hand, if the temperature-elevating rate is 2.5°C./min or less, deposition of Al₂O₃ or the like within the target canpreferably be prevented.

The temperature-elevating rate from 800° C. to a sintering temperatureis preferably 0.1 to 2.0° C./min, more preferably 0.1 to 1.5° C./min.

The temperature-decreasing rate (cooling rate) during calcination isnormally 10° C./min or less, preferably 9° C./min or less, morepreferably 8° C./min or less, further preferably 7° C./min or less, andparticularly preferably 5° C./min or less. If the temperature-decreasingrate is 10° C./min or less, the crystal form of the invention can beobtained easily. In addition, cracks hardly occur during temperaturedecreasing.

In order to allow the bulk resistance of the sintered body obtained inthe above-mentioned calcination step to be uniform in the entire target,a reduction step may be further provided, if necessary.

As the reduction method, a reduction treatment by a reductive gas, areduction treatment by vacuum calcination, a reduction treatment by aninert gas or the like can be given, for example.

In the case of a reduction treatment by a reductive gas, hydrogen,methane, carbon monoxide, or a mixed gas of these gases with oxygen orthe like can be used.

In the case of a reduction treatment by calcinating in an inert gas,nitrogen, argon, or a mixed gas of these gases with oxygen or the likecan be used.

The temperature at the time of the above-mentioned reduction treatmentis normally 100 to 800° C., preferably 200 to 800° C. The reductiontreatment is conducted normally for 0.01 to 10 hours, preferably 0.05 to5 hours.

To sum up, in the method for producing a sintered body used in theinvention, a water-based solvent is compounded with raw material powderscontaining mixed powder of indium oxide power, zinc oxide powder andaluminum oxide powder to obtain a slurry. The obtained slurry is thenmixed for 12 hours or longer, and is subjected to solid-liquidseparation, dried and granulated. Subsequently, the granulated productis put in a mold and formed. Then, the resulting formed body is sinteredat 1200 to 1650° C. for 10 to 50 hours with a temperature-elevating ratein a temperature range of from 800° C. to the sintering temperaturebeing 0.1 to 2.5° C./min and with a temperature-decreasing rate (coolingrate) at the time of calcination being 10° C./min or shorter, wherebythe sintered body of the invention can be obtained.

By processing the sintered body obtained above, the sputtering target ofthe invention can be obtained. Specifically, by grinding the sinteredbody into a shape suited to be mounted in a sputtering apparatus, asputtering target (target material) is obtained. If necessary, thesputtering target material may be bonded to a backing plate to obtain asputtering target.

In order to allow the sintered body to be a target material, thesintered body is ground by means of a surface grinder to allow thesurface roughness Ra to be 0.5 μm or less. Further, the sputteringsurface of the target material may be subjected to mirror finishing,thereby allowing the average surface roughness thereof Ra to be 1000 Åor less.

For this mirror finishing (polishing), known polishing techniques suchas mechanical polishing, chemical polishing, mechano-chemical polishing(combination of mechanical polishing and chemical polishing) or the likemay be used. For example, it can be obtained by polishing by means of afixed abrasive polisher (polishing liquid: water) to attain a roughnessof #2000 or more, or can be obtained by a process in which, afterlapping by a free abrasive lap (polisher: SiC paste or the like),lapping is conducted by using diamond paste as a polisher instead of theSiC paste. There are no specific restrictions on these polishingmethods.

It is preferable to finish the surface of the target material by meansof a #200 to #10,000 diamond wheel, particularly preferably by means ofa #400 to #5,000 diamond wheel. If a diamond wheel with a mesh size of#200 to #10,000 is used, breakage of the target material can beprevented.

It is preferred that the surface roughness Ra of the target material be0.5 μm or less and that the grinding surface have no directivity. If Rais 0.5 μm or less and the grinding surface has no directivity,occurrence of abnormal discharge or generation of particles canpreferably be prevented.

Finally, the thus processed target material is subjected to a cleaningtreatment. For cleaning, air blowing, washing with running water or thelike can be used. When foreign matters are removed by air blowing,foreign matters can be removed more effectively by air intake by meansof a dust collector from the side opposite from the nozzle.

Since the above-mentioned air blow or washing with running water has itslimit, ultrasonic cleaning or the like can also be conducted. Inultrasonic cleaning, it is effective to conduct multiplex oscillationwithin a frequency range of 25 to 300 KHz. For example, it is preferableto perform ultrasonic cleaning by subjecting 12 kinds of frequencycomposed of every 25 KHz in a frequency range of 25 to 300 KHz tomultiplex oscillation.

The thickness of the target material is normally 2 to 20 mm, preferably3 to 12 mm, and particularly preferably 4 to 6 mm.

By bonding the target material obtained in the manner as mentioned aboveto a backing plate, a sputtering target can be obtained. A plurality oftarget materials may be provided in a single backing plate to be used asa substantially single target.

II. Oxide Semiconductor Thin Film

By forming the sputtering target of the invention into a film by asputtering method, the oxide semiconductor thin film of the inventioncan be obtained.

The oxide semiconductor thin film of the invention is composed ofindium, tin, zinc, aluminum and oxygen and preferably satisfies thefollowing atomic ratios (1) to (4):

0.10≦In/(In+Sn+Zn+Al)≦0.75  (1)

0.01≦Sn/(In+Sn+Zn+Al)≦0.30  (2)

0.10≦Zn/(In+Sn+Zn+Al)≦0.70  (3)

0.01≦Al/(In+Sn+Zn+Al)≦0.40  (4)

wherein In, Sn, Zn and Al independently indicate the atomic ratio ofeach element in the sputtering target.

In the formula (1), if the atomic ratio of the In element is 0.10 ormore, the degree of overlapping of the In 5s orbital can be kept large,and as a result, the field effect mobility may be 10 cm²Ns or moreeasily. On the other hand, if the atomic ratio of the In element 0.75 orless, good reliability can be obtained if the formed film is applied toa channel layer of a TFT

In the formula (2), if the atomic ratio of the Sn element is 0.01 ormore, an increase in target resistance can be suppressed, and filmformation can be stabilized easily since abnormal discharge hardlyoccurs during sputtering. On the other hand, if the atomic ratio of theSn element is 0.30 or less, lowering in solubility of the resulting thinfilm in a wet etchant can be prevented, whereby wet etching can beconducted without problems.

In the formula (3), if the amount of the Zn element is 0.10 or more, theresulting film can be stable as an amorphous film. On the other hand, ifthe amount of the Zn element is 0.70 or less, since the dissolutionspeed of the resulting thin film in a wet etchant is not too high,resulting in smooth wet etching.

In the formula (4), if the atomic ratio of the Al element is 0.01 ormore, an increase in the oxygen partial pressure during film formationcan be prevented. Since the Al element is bonded to oxygen strongly, itcan lower the oxygen partial pressure during film formation. Further,when a channel layer is formed and applied to a TFT, good reliabilitycab be obtained. On the other hand, if the atomic ratio of the Alelement is 0.40 or less, occurrence of abnormal discharge due toformation of Al₂O₃ at the time of film formation by sputtering can beprevented, leading to stable film formation.

The carrier concentration of the oxide semiconductor thin film isnormally 10¹⁹/cm³ or less, preferably 10¹³ to 10¹⁸/cm³, furtherpreferably 10¹⁴ to 10¹⁸/cm³, and particularly preferably 10¹⁵ to10¹⁸/cm³.

If the carrier concentration of the oxide layer is 10¹⁹ cm³ or less, itis possible to prevent current leakage, normally-on, lowering in on-offratio when a device such as a thin film transistor is fabricated. As aresult, good transistor performance can be exhibited. Further, if thecarrier concentration is 10¹³ cm³ or more, the device can be driven as aTFT without causing problems.

The carrier concentration of the oxide semiconductor thin film can bemeasured by the Hall effect measurement. Specifically, it can bemeasured by the methods described in the Examples.

Due to a high conductivity, a DC sputtering method having a highfilm-forming speed can be applied to the sputtering target of theinvention.

In addition to the above-mentioned DC sputtering method, the RFsputtering method, the AC sputtering method and the pulse DC sputteringmethod can be applied to the sputtering target of the invention, andsputtering free from abnormal discharge can be conducted.

The oxide semiconductor thin film can also be formed by using theabove-mentioned sintered body by the deposition method, the ion-platingmethod, the pulse laser deposition method or the like in addition to thesputtering method.

As the sputtering gas (atmosphere) in the production of the oxidesemiconductor thin film of the invention, a mixed gas of a rare gas suchas argon and an oxidizing gas can be used. Examples of the oxidizing gasinclude O₂, CO₂, O₃, H₂O and N₂O. As the sputtering gas, a mixed gascontaining a rare gas, and one or more gases selected from water vapor,an oxygen gas and a nitrous oxide gas is preferable. A mixed gascontaining a rare gas and at least water vapor is more preferable.

The oxygen partial pressure ratio at the time of film formation bysputtering is preferably 0% or more and less than 40%. A thin filmformed under the conditions in which the oxygen partial pressure ratiois less than 40% may not have a significantly decreased carrierconcentration. As a result, it becomes possible to prevent the carrierconcentration from being less than 10¹³ cm³.

The oxygen partial pressure ratio is preferably 0% to 30% andparticularly preferably 0% to 20%.

The partial pressure ratio of water vapor contained in a sputtering gas(atmosphere) at the time of depositing an oxide thin film in theinvention, i.e. [H₂O]/([H₂O]+[rare gas]+[other gases]), is preferably0.1 to 25%. If the water partial pressure is 25% or less, it is possibleto prevent a decrease in film density, and as a result, the degree ofoverlapping of the In 5s orbital can be kept large, whereby lowering inmobility can be prevented.

The partial pressure ratio of water in the atmosphere at the time ofsputtering is more preferably 0.7 to 13%, with 1 to 6% beingparticularly preferable.

The substrate temperature at the time of film formation by sputtering ispreferably 25 to 120° C., further preferably 25 to 100° C., andparticularly preferably 25 to 90° C.

If the substrate temperature at the time of film formation is 120° C. orless, oxygen or the like can be incorporated sufficiently at the time offilm formation, whereby an excessive increase in carrier concentrationof the thin film after heating can be prevented. Further, if thesubstrate temperature at the time of film formation is 25° C. or more,the denseness of the thin film may not be lowered, and as a result,lowering in mobility of a TFT can be prevented.

It is preferred that the oxide thin film obtained by sputtering befurther subjected to an annealing treatment by retaining at 150 to 500°C. for 15 minutes to 5 hours. The annealing treatment temperature afterfilm formation is more preferably 200° C. or more and 450° C. or less,further preferably 250° C. or more and 350° C. or less. By conductingthe above-mentioned annealing treatment, semiconductor properties can beobtained.

The heating atmosphere is not particularly restricted. In respect ofcarrier control properties, the air atmosphere or the oxygen-circulatingatmosphere is preferable.

In the annealing treatment as the post treatment of the oxide thin film,in the presence or absence of oxygen, a lamp annealing apparatus, alaser annealing apparatus, a thermal plasma apparatus, a hot air heatingapparatus, a contact heating apparatus or the like can be used.

The distance between the target and the substrate at the time ofsputtering is preferably 1 to 15 cm in a direction perpendicular to thefilm-forming surface of the substrate, with 2 to 8 cm being furtherpreferable.

If this distance is 1 cm or more, the kinetic energy of particles oftarget-constituting elements which arrive at the substrate can beprevented from becoming excessively large, and hence, good filmproperties can be obtained. Further, in-plane distribution or the likeof the film thickness and the electric characteristics can be prevented.

If the distance between the target and the substrate is 15 cm or less,the kinetic energy of particles of target-constituting elements can beprevented from becoming too small, and a dense film may be obtained, andas a result, good semiconductor properties can be obtained.

As for the formation of an oxide thin film, it is desirable that filmformation be conducted by sputtering in an atmosphere having a magneticfield intensity of 300 to 1500 gausses. If the magnetic field intensityis 300 gausses or more, since lowering in plasma density can beprevented, sputtering can be conducted without problems even if thesputtering target has a high resistance. On the other hand, if themagnetic field intensity is 1500 gausses or less, deterioration incontrollability of the film thickness and the electric characteristicsof the film can be suppressed.

No specific restrictions are imposed on the pressure of a gas atmosphere(sputtering pressure), as long as plasma is stably discharged. Thepressure is preferably 0.1 to 3.0 Pa, further preferably 0.1 to 1.5 Pa,with 0.1 to 1.0 Pa being particularly preferable. If the sputteringpressure is 3.0 Pa or less, the mean free path of sputtering particlesis not shortened excessively, thereby preventing lowering in density ofa thin film. If the sputtering pressure is 0.1 Pa or more, fine crystalscan be prevented from being formed in a film during film formation.

Meanwhile, the sputtering pressure is the total pressure in the systemat the start of sputtering after rare gas atoms (e.g. argon), watermolecules, oxygen molecules or the like are introduced.

The formation of an oxide semiconductor thin film may be conducted bythe following AC sputtering.

Substrates are transported in sequence to positions opposing to three ormore targets arranged in parallel with a prescribed interval in a vacuumchamber. Then, a negative potential and a positive potential are appliedalternately from an AC power source to each of the targets, wherebyplasma is caused to be generated on the target and a film is formed onthe surface of the substrate.

At this time, film formation is conducted by applying at least oneoutput from an AC power source while switching the target to which apotential is applied among two or more targets connected divergently.That is, at least one output from the AC power source is connecteddivergently to two or more targets respectively, whereby film formationis conducted while applying different potentials to the adjacenttargets.

If an oxide semiconductor thin film is formed by AC sputtering, it ispreferred that sputtering be conducted in an atmosphere of a mixed gascontaining a rare gas, and one or more gases selected from water vapor,an oxygen gas and a nitrous oxide gas, for example. It is particularlypreferred that sputtering be conducted in an atmosphere of a mixed gascontaining water vapor.

If film formation is conducted by AC sputtering, not only it is possibleto obtain an oxide layer which has excellent large-area uniformity onthe industrial basis, but also it can be expected that the useefficiency of the target is increased.

If a film is formed by sputtering on a large-area substrate in which thelength of one side exceeds 1 m, it is preferable to use an AC sputteringapparatus for producing a large-area film such as that disclosed inJP-A-2005-290550.

The AC sputtering apparatus disclosed in JP-A-2005-290550 specificallyhas a vacuum chamber, a substrate holder arranged within the vacuumchamber and a sputtering source arranged at a position opposing to thissubstrate holder. FIG. 1 shows essential parts of a sputtering source ofthe AC sputtering apparatus. The sputtering source has a plurality ofsputtering parts, which respectively have plate-like targets 31 a to 31f. Assuming that the surface to be sputtered of each target 31 a to 31 fis a sputtering surface, the sputtering parts are arranged such that thesputtering surfaces are on the same plane. Targets 31 a to 31 f areformed in a long and narrow form having a longitudinal direction, andthey have the same shape. The targets are arranged such that the edgeparts (side surface) in the longitudinal direction of the sputteringsurface are arranged in parallel with a prescribed intervaltherebetween. Accordingly, the side surfaces of the adjacent targets 31a to 31 f are in parallel.

Outside the vacuum chamber, AC power sources 17 a to 17 c are arranged.Among the two terminals of each of AC power sources 17 a to 17 c, oneterminal is connected to one electrode of the adjacent two electrodes,and the other terminal is connected to the other electrode. The twoterminals of each AC power source 17 a to 17 c output voltages differingin polarity (positive and negative), and the targets of 31 a to 31 f arefitted in close contact with the electrode, whereby, to adjacent twotargets 31 a to 31 f, an alternate voltage differing in polarity isapplied from the AC power sources 17 a to 17 c. Therefore, among theadjacent targets of 31 a to 31 f, if one is set in a positive potential,the other is set in a negative potential.

On the side opposite to the targets 31 a to 31 f of the electrode,magnetic field forming means 40 a to 40 f are arranged. Each magneticfield forming means 40 a to 40 f has a long and narrow ring-like magnethaving an approximately same size as that of the outer circumference ofthe targets 31 a to 31 f, and a bar-like magnet which is shorter thanthe length of the ring-like magnet.

Each ring-like magnet is arranged at the position right behind onecorresponding target 31 a to 31 f such that the ring-like magnets arearranged in parallel with the longitudinal direction of the targets 31 ato 31 f. As mentioned above, since the targets 31 a to 31 f are arrangedin parallel with a prescribed interval therebetween, the ring-likemagnets are arranged with the same interval as that for the targets 31 ato 31 f from each other.

The AC power density when an oxide target is used in AC sputtering ispreferably 3 W/cm² or more and 20 W/cm² or less. If the power density is3 W/cm² or more, the film-forming speed does not become too slow, andensures economical advantage in respect of production. A power densityof 20 W/cm² or less can prevent breakage of the target. A morepreferable power density is 3 W/cm² to 15 W/cm².

The frequency of the AC sputtering is preferably in a range of 10 kHz to1 MHz. If the frequency is 10 kHz or more, noise problems hardly occur.If the frequency is 1 MHz or less, sputtering in other places than thedesired target position due to excessively wide scattering of plasma canbe prevented from being conducted, whereby uniformity can be kept. Amore preferable AC sputtering frequency is 20 kHz to 500 kHz.

Conditions or the like at the time of sputtering other than thosementioned above may be appropriately selected from the conditions givenabove.

III. Thin Film Transistor

The above-mentioned oxide semiconductor thin film can be used in a thinfilm transistor (TFT). It can be used particularly preferably as achannel layer.

No specific restrictions are imposed on the device configuration of thethin film transistor of the invention, as long as it has theabove-mentioned oxide thin film as a channel layer. Known various deviceconfigurations can be used.

By using the oxide thin film mentioned above as a channel layer of aTFT, a TFT having a high field effect mobility and high reliability canbe obtained. The TFT of the invention preferably has a field effectmobility of 10 cm²Ns or more, more preferably 13 cm²Ns or more. Thefield effect mobility can be measured by the method described in theExamples.

The film thickness of the channel layer in the thin film transistor ofthe invention is normally 10 to 300 nm, preferably 20 to 250 nm, morepreferably 30 to 200 nm, further preferably 35 to 120 nm, andparticularly preferably 40 to 80 nm. If the film thickness of thechannel layer is 10 nm or more, the film thickness does not becomenon-uniform easily even when the film is formed to have a large area,the properties of a TFT fabricated may become uniform within the plane.If the film thickness is 300 nm or less, the film formation time is notexcessively prolonged.

The channel layer in the thin film transistor of the invention isnormally used in the N-type region. However, in combination with variousP-type semiconductors such as a P-type Si-based semiconductor, a P-typeoxide semiconductor and a P-type organic semiconductor, the channellayer can be used in various semiconductor devices such as a PN junctiontransistor.

In the thin film transistor of the invention, it is preferred that aprotective film be provided on the channel layer. It is preferred thatthe protective film in the thin film transistor of the inventioncomprise at least SiN_(x). As compared with SiO₂, SiN_(x) is capable offorming a dense film, and hence has an advantage that it has significanteffects of preventing deterioration of a TFT.

The protective film may comprise, in addition to SiNx, an oxide such asSiO₂, Al₂O₃, Ta₂O₅, TiO₂, MgO, ZrO₂, CeO₂, K₂O, Li₂O, Na₂O, Rb₂O, Sc₂O₃,Y₂O₃, HfO₂, CaHfO₃, PbTiO₃, BaTa₂O₆, Sm₂O₃, SrTiO₃ or AlN.

As for the oxide thin film of the invention that comprises an indium(In) element, a tin (Sn) element, a zinc (Zn) element and an aluminum(Al) element, since it contains Al, resistance to reduction by the CVDprocess is improved. As a result, the back channel side is hardlyreduced by a process in which a protective film is prepared, wherebySiN_(x) can be used as a protective film.

Before forming a protective film, it is preferred that the channel layerbe subjected to an ozone treatment, an oxygen plasma treatment, anitrogen dioxide plasma treatment or a nitrous oxide plasma treatment.Such a treatment may be conducted at any time as long as it is after theformation of a channel layer and before the formation of a protectivefilm. However, it is desirable that the treatment be conductedimmediately before the formation of a protective film. By conductingsuch a pre-treatment, generation of oxygen deficiency in the channellayer can be suppressed.

If hydrogen in the oxide semiconductor film diffuses during driving of aTFT, the threshold voltage may be shifted, resulting in lowering ofreliability of a TFT. By subjecting the channel layer to an ozonetreatment, an oxygen plasma treatment or a nitrous oxide plasmatreatment, the In—OH bonding in the thin film structure is stabilized,whereby diffusion of hydrogen in the oxide semiconductor film can besuppressed.

The thin film transistor normally comprises a substrate, a gateelectrode, a gate insulating layer, an organic semiconductor layer(channel layer), a source electrode and a drain electrode. The channellayer is as mentioned above. A known material can be used for thesubstrate.

No particular restrictions are imposed on the material forming the gateinsulating film in the thin film transistor of the invention. A materialwhich is generally used can be arbitrarily selected. Specifically, acompound such as SiO₂, SiN_(X), Al₂O₃, Ta₂O₅, TiO₂, MgO, ZrO₂, CeO₂,K₂O, Li₂O, Na₂O, Rb₂O, Sc₂O₃, Y₂O₃, HfO₂, CaHfO₃, PbTiO₃, BaTa₂O₆,SrTiO₃, Sm₂O₃, AlN or the like can be used, for example. Among these,SiO₂, SiN_(X), Al₂O₃, Y₂O₃, HfO₂ and CaHfO₃ are preferable, with SiO₂,SiN_(X), HfO₂ and Al₂O₃ being more preferable.

The gate insulating film can be formed by the plasma CVD (Chemical VaporDeposition) method, for example.

If a gate insulating film is formed by the plasma CVD method and achannel layer is formed thereon, hydrogen in the gate insulating filmdiffuses in the channel layer, and as a result, deterioration of filmquality of the channel layer or lowering of reliability of a TFT may becaused. In order to prevent deterioration of film quality of the channellayer or lowering of reliability of a TFT, it is preferred that the gateinsulating film be subjected to an ozone treatment, an oxygen plasmatreatment, a nitrogen dioxide plasma treatment or a nitrous oxide plasmatreatment before the formation of a channel layer. By conducting such apre-treatment, deterioration of film quality of the channel layer orlowering of reliability of a TFT can be prevented.

The number of oxygen atoms of these oxides does not necessarily coincidewith the stoichiometric ratio. For example, SiO₂ or SiO_(x) may be used.

The gate insulting film may have a structure in which two or moreinsulating films composed of different materials are stacked. The gateinsulating film may be crystalline, polycrystalline, or amorphous. Thegate insulating film is preferably polycrystalline or amorphous from theviewpoint of easiness of industrial production.

No specific restrictions are imposed on the material forming eachelectrode in the thin film transistor, i.e. a drain electrode, a sourceelectrode and a gate electrode, and materials which are generally usedcan be arbitrarily selected. For example, transparent electrodes such asITO, IZO, ZnO, SnO₂ or the like, a metal electrode such as Al, Ag, Cu,Cr, Ni, Mo, Au, Ti, and Ta or an alloy metal electrode containing thesemetals can be used.

Each of the drain electrode, the source electrode and the gate electrodemay have a multi-layer structure in which two or more differentconductive layers are stacked. In particular, since the source/drainelectrodes are required to be used in low-resistance wiring, theelectrodes may be used by sandwiching a good conductor such as Al and Cubetween metals having good adhesiveness such as Ti and Mo.

The thin film transistor of the invention can be applied to variousintegrated circuits such as a field effect transistor, a logicalcircuit, a memory circuit and a differential amplifier circuit. Further,in addition to a field effect transistor, it can be applied to a staticinduction transistor, a Schottky barrier transistor, a Schottky diodeand a resistance element.

As for the configuration of the thin film transistor of the invention, aknown configuration such as a bottom-gate configuration, abottom-contact configuration and a top-contact configuration can be usedwithout restrictions.

In particular, a bottom-gate configuration is advantageous since highperformance can be obtained as compared with a thin film transistorcomprising amorphous silicon or ZnO. The bottom-gate configuration ispreferable since the number of masks at the time of production can bedecreased easily and the production cost for application such as alarge-sized display or the like can be reduced easily.

The thin film transistor of the invention can preferably be used as adisplay.

For use in a large-sized display, a channel-etch type bottom-gate thinfilm transistor is particularly preferable. A channel-etch typebottom-gate thin film transistor can produce a panel for a display at alow cost since the number of photo-masks used in photolithography issmall. Among these, a channel-etch type thin film transistor having abottom-gate configuration and a channel-etch type thin film transistorhaving a top-contact configuration are particularly preferable sincethey have excellent properties such as mobility and can beindustrialized easily.

EXAMPLES Examples 1 to 6 Production of Oxide Sintered Body

As raw material powders, the following oxide powders were used. Themedian size D50 was employed as an average particle size for thefollowing oxide powders. The average particle size was measured by alaser diffraction particle size analyzer SALD-300V (manufactured byShimadzu Corporation).

Indium oxide powder: average particle size 0.98 μm

Tin oxide powder: average particle size 0.98 μm

Zinc oxide powder: average particle size 0.96 μm

Aluminum oxide powder: average particle size 0.98 μm

The above-mentioned powders were weighed such that the atomic ratioshown in Table 1 was attained. They were finely pulverized and mixeduniformly and then granulated by adding a binder for forming.Subsequently, the mixed powder of the raw materials was filled in themold uniformly and press-formed at a pressing pressure of 140 MPa in acold press apparatus.

The formed body obtained was sintered in a sintering furnace at atemperature-elevating rate (from 800° C. to the sintering temperature),a sintering temperature, a sintering time and a temperature-decreasingrate shown in Table 1 to produce a sintered body. During the temperatureelevation, the atmosphere was oxygen, and otherwise air (atmosphere).

TABLE 1 Temperature- elevating rate [° C./min] Temperature- Ratio ofmetal element (from 800° C. to Sintering Sintering decreasing (X)[X/(In + Sn + Zn + Al)] sintering temperature time rate (cooling rate) X= In X = Sn X = Zn X = Al temperature) [° C.] [hr] [° C./min] Ex. 1 0.500.15 0.25 0.10 0.60 1450 20 10 Ex. 2 0.70 0.05 0.20 0.05 0.60 1450 20 10Ex. 3 0.50 0.10 0.30 0.10 0.60 1450 20 10 Ex. 4 0.50 0.05 0.35 0.10 0.601450 20 10 Ex. 5 0.50 0.10 0.30 0.10 0.10 1470 20 8 Ex. 6 0.50 0.05 0.350.10 0.10 1470 20 8 Comp. Ex. 1 0.85 0.10 0.045 0.005 5.5 1150 8 15Comp. Ex. 2 0.80 0.13 0.065 0.005 5.5 1150 8 15

[Analysis of Sintered Body]

The relative density of the sintered body obtained was measured by theArchimedean method. The sintered bodies of Examples 1 to 6 wereconfirmed to have a relative density of 98% or more.

Further, the bulk specific resistance (conductivity) of the sinteredbody obtained was measured by means of a resistivity meter (Loresta,manufactured by Mitsubishi Chemical Analytech Co., Ltd.) in accordancewith the four point probe method (JIS R 1637). The results are shown inTable 1. As shown in Table 1, the bulk specific resistances of thesintered bodies of Examples 1 to 6 were 10 mΩcm or less.

For the sintered bodies obtained, ICP-AES analysis was conducted. As aresult, they were confirmed to have the atomic ratios shown in Table 1.

Moreover, the crystal structure of each of the sintered body obtainedwas determined by means of an X-ray diffraction (XRD) apparatus. X-raydiffraction charts of the sintered bodies obtained in Examples 1 to 6are shown in FIGS. 2 to 7, respectively.

The formed compounds contained in the sintered body (crystal structureof the oxide) are shown in Table 2.

TABLE 2 Relative Bulk specific Occurrence of Number of Compounds formeddensity resistance abnormal discharge nodules generated in sintered body[%] [mΩcm] during sputtering [number/3 mm²] Example 1 InAlZnO₄, In₂O₃99.5 1.5 None 0 Example 2 InAlZn₂O₅, In₂O₃ 98.9 3.3 None 0 Example 3InAlZn₂O₅, InAlZnO₄, In₂O₃ 99.2 2.8 None 0 Example 4 InAlZn₂O₅, In₂O₃99.0 2.3 None 0 Example 5 InAlZn₂O₅, InAlZnO₄, In₂O₃ 99.9 1.0 None 0Example 6 InAlZn₂O₅, In₂O₃ 99.9 1.2 None 0 Comp. Ex. 1 In₂O₃, Zn₂SnO₄95.2 18.5 Micro arc generated 15 Comp. Ex. 2 In₂O₃, Zn₂SnO₄ 96.8 20.9Micro arc generated 27

As a result of analyzing the chart, a homologous structure of InAlZnO₄and a bixbyite structure of In₂O₃ were observed in the sintered body ofExample 1. The crystal structure was confirmed by using JCPDS cards andICSD.

The homologous structure represented by InAlZnO₄ has a peak pattern ofNo. 40-0258 of the JCPDS database. The bixbyite structure represented byIn₂O₃ has a peak pattern of No. 06-0416 of the JCPDS data base.

In the sintered body of Example 2, the homologous structure representedby InAlZn₂O₅ and the bixbyite structure represented by In₂O₃ wereobserved. The homologous structure represented by InAlZn₂O₅ has a peakpattern of No. 40-0259 of the JCPDS data base.

In the sintered body of Example 3, the homologous structure representedby InAlZn₂O₅, the homologous structure represented by InAlZnO₄ and thebixbyite structure represented by In₂O₃ were observed.

In the sintered body of Example 4, the homologous structure representedby InAlZn₂O₅ and the bixbyite structure represented by In₂O₃ wereobserved.

In the sintered body of Example 5, the homologous structure representedby InAlZn₂O₅, the homologous structure represented by InAlZnO₄ and thebixbyite structure represented by In₂O₃ were observed.

In the sintered body of Example 6, the homologous structure representedby InAlZn₂O₅ and the bixbyite structure represented by In₂O₃ wereobserved.

The spinel structure represented by Zn₂SnO₄ was not observed in thesintered bodies of Examples 1 to 6.

The X-ray diffraction measurement (XRD) was measured under the followingconditions.

-   -   Apparatus: Ultima-III manufactured by Rigaku Corporation    -   X-ray: Cu-Kα rays (wavelength 1.5406 Å, monochromatized with a        graphite monochromator)    -   2θ-θ reflection method, continuous scanning (1.0°/min)    -   sampling interval: 0.02°    -   slits DS, SS: ⅔°, RS: 0.6 mm

For the sintered bodies of Examples 1 to 6, the dispersion of Sn or Alin the sintered body obtained by the electron probe microanalyzer (EPMA)measurement was checked. An 8 μm or larger-sized aggregation of Sn or Alwas not observed. As a result, the sputtering targets of the inventionwere found to be significantly excellent in dispersivity andhomogeneousness. The EPMA measurement was conducted under the followingconditions.

Apparatus: JXA-8200 (JEOL Ltd.)

Accelerating voltage: 15 kV

Irradiation current: 50 nA

Irradiation time (per one point): 50 mS

[Production of Sputtering Target]

The surface of the sintered bodies obtained in Examples 1 to 6 wasground by means of a surface grinder. The sides thereof were cut using adiamond cutter. The sintered bodies were bonded to a backing plate,thereby to obtain sputtering targets each having a diameter of 4 inches.Further, in Examples 1, 3 and 4, targets each having a width of 200 mm,a length of 1700 mm and a thickness of 10 mm were fabricated for ACsputtering film-forming (Examples 13 to 15, mentioned later).

[Confirmation of Presence or Absence of Abnormal Discharge]

A sputtering target having a diameter of 4 inches obtained was mountedin a DC sputtering apparatus. As the atmosphere, a mixed gas obtained byadding a H₂O gas to an argon gas at a partial pressure ratio of 2% wasused. A 10 kWh continuous sputtering was conducted at a DC power of 400W with the sputtering pressure being 0.4 Pa and the substratetemperature being room temperature. Variations in voltage during thesputtering were stored in data logger to confirm the presence or absenceof abnormal discharge. The results are shown in Table 2.

The above-mentioned presence or absence of abnormal discharge wasdetermined by detecting abnormal discharge while monitoring variationsin voltage. Specifically, “abnormal discharge” is defined by the casewhere the voltage variation generated for a measurement time of 5minutes is 10% or more of the working voltage during sputteringoperation. In particular, when the working voltage during a sputteringoperation varies within a range of ±10% for 0.1 second, micro arcs,which are abnormal discharge of sputtering discharge, may generate,thereby lowering the yield of a device. Accordingly, they may beunsuitable for mass production.

[Confirmation of Presence or Absence of Nodule Generation]

Sputtering was conducted continuously for 40 hours by using a4-inch-diameter sputtering target obtained with the atmosphere being amixed gas obtained by adding a hydrogen gas to an argon gas at a partialpressure ratio of 3% to confirm the presence or absence of nodulegeneration. As a result, on the surface of sputtering targets ofExamples 1 to 6, no nodules were observed.

The conditions at the time of the sputtering include a sputteringpressure of 0.4 Pa, a DC power of 100 W and a substrate temperature ofroom temperature. The hydrogen gas was added to the atmosphere gas inorder to promote nodule generation.

For evaluation of nodules, the following method was employed. The changein the target surface after sputtering was observed at a magnificationof 50 times by means of a stereomicroscope. The number average ofnodules having a size of 20 μm or larger generated in the visual fieldof 3 mm² was calculated. Table 2 shows the number of nodules generated.

Comparative Examples 1 and 2

Sintered bodies and sputtering targets were produced and evaluated inthe same manner as in Example 1, except that the raw material powderswere mixed according to the atomic ratio shown in Table 1, and sinteredat a temperature-elevating rate (from 800° C. to the sinteringtemperature), at a sintering temperature, for a sintering time shown inTable 1 and at a temperature-decreasing rate of 15° C./min. The resultsare shown in Tables 1 and 2.

In the targets of Comparative Examples 1 and 2, the homologous structurecompound represented by InAlO₃(ZnO)_(m) (m is 0.1 to 10) was notobserved, and the bixbyite structure represented by In₂O₃ and the spinelstructure represented by Zn₂SnO₄ were observed. The spinel structurerepresented by Zn₂SnO₄ has a peak pattern of No. 24-1470 of the JCPDScard.

In the sputtering targets of Comparative Examples 1 and 2, abnormaldischarge occurred during sputtering, and nodules were observed on thetarget surface.

It was revealed that, in the sintered bodies of Comparative Examples 1and 2, since both of the homologous structure compound represented byInAlO₃(ZnO)_(m) (m is 0.1 to 10) and the bixbyite structure representedby In₂O₃ were not formed, the densities of the sintered bodies werelowered, resulting in an increase in bulk resistance. It can beconsidered that nodules were generated due to such an increase in bulkresistance.

Examples 7 to 12 Production of Oxide Semiconductor Thin Film

The 4-inch targets produced in Examples 1 to 6 and having thecompositions shown in Table 3 or 4 were mounted in a magnetronsputtering apparatus, and slide glass (#1737, manufactured by CorningInc.) was installed as a substrate in each case. By the DC magnetronsputtering method, a 50 nm-thick amorphous film was formed on the slideglass under the following conditions. At the time of film formation, anAr gas, an O₂ gas and a H₂O gas were introduced at partial pressureratios (%) shown in Table 3 or 4. The substrate on which an amorphousfilm had been formed was heated in an atmosphere at 300° C. for 60minutes.

Sputtering conditions were as follows.

Substrate temperature: 80° C.

Ultimate pressure: 8.5×10⁻⁵ Pa

Atmospheric gas: Ar gas, O₂ gas, H₂O gas (for partial pressure, seeTables 3 and 4)

Sputtering pressure (total pressure): 0.4 Pa

Input power: DC 100 W

S (substrate)-T (target) distance: 70 mm

TABLE 3 Example 7 Example 8 Example 9 Example 10 Target compositionIn/(In + Sn + In/(In + Sn + In/(In + Sn + In/(In + Sn + Zn + Al) = 0.50Zn + Al) = 0.70 Zn + Al) = 0.50 Zn + Al) = 0.50 Sn/(In + Sn + Sn/(In +Sn + Sn/(In + Sn + Sn/(In + Sn + Zn + Al) = 0.15 Zn + Al) = 0.05 Zn +Al) = 0.10 Zn + Al) = 0.05 Zn/(In + Sn + Zn/(In + Sn + Zn/(In + Sn +Zn/(In + Sn + Zn + Al) = 0.25 Zn + Al) = 0.20 Zn + Al) = 0.30 Zn + Al) =0.35 Al/(In + Sn + Al/(In + Sn + Al/(In + Sn + Al/(In + Sn + Zn + Al) =0.10 Zn + Al) = 0.05 Zn + Al) = 0.10 Zn + Al) = 0.10 Sputtering Ultimatepressure (Pa) 8.5 × 10⁻⁵ 8.5 × 10⁻⁵ 8.5 × 10⁻⁵ 8.5 × 10⁻⁵ conditionsSputtering pressure (Pa) 0.4 0.4 0.4 0.4 [H₂O]/([H₂O] + [Ar] + [O₂]) (%)0 2 1 0 [Ar]/([H₂O] + [Ar] + [O₂]) (%) 85 78 94 75 [O₂]/([H₂O] + [Ar] +[O₂]) (%) 15 20 5 25 Water partial pressure (Pa)  0.0E+00  8.0E−03 4.0E−03  0.0E+00 Sputtering method DC DC DC DC T-S distance (mm) 70 7070 70 Film thickness (nm) 50 50 50 50 Substrate temperature (° C.) 80 8080 80 Annealing Annealing temperature (° C.) 300 300 300 300 Annealingtime (min) 60 60 60 60 Atmosphere Air Air Air Air Hall Carrierconcentration (cm⁻³) 5.68E+17 3.05E+17 3.29E+17 6.91E+17 measurement TFTChannel width/Channel length (μm) 20/10 20/10 20/10 20/10 Source/drainMo Mo Mo Mo Source/drain patterning Lift off Lift off Lift off Lift offChannel treatment before Nitrous oxide plasma Nitrous oxide plasmaNitrous oxide plasma Nitrous oxide plasma forming of protective filmProtective film SiOx/SiNx SiOx/SiNx SiOx/SiNx SiOx/SiNx Mobility(cm²/Vs) 17.5 14.9 17.0 20.3 Threshold voltage Vth (V) 1.8 0.26 1.5 0.17S value (V/dec) 0.16 0.21 0.12 0.20 Threshold voltage shift ΔVth(V) 0.190.27 0.18 0.22

TABLE 4 Example 11 Example 12 Comp. Ex. 3 Comp. Ex. 4 Target compositionIn/(In + Sn + In/(In + Sn + In/(In + Sn + In/(In + Sn + Zn + Al) = 0.50Zn + Al) = 0.50 Zn + Al) = 0.85 Zn + Al) = 0.80 Sn/(In + Sn + Sn/(In +Sn + Sn/(In + Sn + Sn/(In + Sn + Zn + Al) = 0.10 Zn + Al) = 0.05 Zn +Al) = 0.10 Zn + Al) = 0.13 Zn/(In + Sn + Zn/(In + Sn + Zn/(In + Sn +Zn/(In + Sn + Zn + Al) = 0.30 Zn + Al) = 0.35 Zn + Al) = 0.045 Zn + Al)= 0.065 Al/(In + Sn + Al/(In + Sn + Al/(In + Sn + Al/(In + Sn + Zn + Al)= 0.10 Zn + Al) = 0.10 Zn + Al) = 0.005 Zn + Al) = 0.005 SputteringUltimate pressure (Pa) 8.5 × 10⁻⁵ 8.5 × 10⁻⁵ 8.5 × 10⁻⁵ 8.5 × 10⁻⁵conditions Sputtering pressure (Pa) 0.4 0.4 0.4 0.4 [H₂O]/([H₂O] +[Ar] + [O₂]) (%) 0 2 0 0 [Ar]/([H₂O] + [Ar] + [O₂]) (%) 70 98 50 50[O₂]/([H₂O] + [Ar] + [O₂]) (%) 30 0 50 50 Water partial pressure (Pa) 0.0E+00  8.0E−03  0.0E+00  0.0E+00 Sputtering method DC DC DC DC T-Sdistance (mm) 70 70 70 70 Film thickness (nm) 50 50 50 50 Substratetemperature (° C.) 80 80 80 80 Annealing Annealing temperature (° C.)300 300 300 300 Annealing time (min) 60 60 60 60 Atmosphere Air Air AirAir Hall Carrier concentration (cm⁻³) 5.80E+17 5.02E+17 5.39E+184.93E+18 measurement TFT Channel width/Channel length (μm) 20/10 20/1020/10 20/10 Source/drain Mo Mo Mo Mo Source/drain patterning Lift offLift off Lift off Lift off Channel treatment before Nitrous oxide plasmaNitrous oxide plasma No treatment No treatment forming protective filmProtective film SiOx/SiNx SiOx/SiNx SiOx/SiNx SiOx/SiNx Mobility(cm²/Vs) 23.2 20.2 28.5 30.8 Threshold voltage Vth (V) 0.35 0.96 −12 −10S value (V/dec) 0.12 0.17 0.55 0.47 Threshold voltage shift ΔVth (V)0.18 0.23 4.9 4.1

[Evaluation of Oxide Semiconductor Thin Film]

A glass substrate on which an oxide semiconductor film had been formedwas set in a Resi Test 8300 (manufactured by TOYO Corporation), and theHall effect was evaluated at room temperature. Specifically, a carrierconcentration was determined. The results are shown in Tables 3 and 4.

Further, by the ICP-AES analysis, it was confirmed that the atomic ratioof each element contained in the oxide thin film was the same as that ofthe sputtering target.

The crystal structure of the oxide thin film formed on the glasssubstrate was examined by means of an X-ray diffraction measurementapparatus (Ultima-III, manufactured by Rigaku Corporation). In Examples7 to 12, no diffraction peaks were observed immediately after thedeposition of the thin film, and hence it was confirmed that the thinfilm was amorphous. After conducting a heat treatment (annealing) in theair at 300° C. for 60 minutes, no diffraction peaks were observed, andthe thin film was confirmed to be amorphous.

The measuring conditions of the XRD are as follows.

Apparatus: Ultima-Ill, manufactured by Rigaku Corporation

X ray: Cu-Kα rays (wavelength: 1.5406 Å, monochromatized by means of agraphite monochrometer)

2θ-θ reflection method, continuous scanning (1.0°/min)

Sampling interval: 0.02°

Slit DS, SS: ⅔°, RS: 0.6 mm

[Production of Thin Film Transistor]

As a substrate, a conductive silicon substrate provided with a 100nm-thick thermally oxidized film was used. The thermally oxidized filmfunctioned as a gate insulating film and the conductive silicon partfunctioned as a gate electrode.

On the gate insulating film, a film was formed by sputtering under theconditions shown in Tables 3 and 4, whereby a 50 nm-thick amorphous thinfilm was formed. As a resist, OFPR#800 (manufactured by Tokyo Ohka KogyoCo., Ltd.) was used. Coating, pre-baking (80° C., 5 minutes) andexposure were conducted. After development, post-baking (120° C., 5minutes), etching with oxalic acid, and patterning into a desired shapewere conducted. Thereafter, the film was subjected to a heat treatmentat 300° C. for 60 minutes in a hot-air oven (annealing treatment).

Thereafter, Mo (100 nm) was formed into a film by sputtering, andsource/drain electrodes were patterned by the lift-off method in adesired shape. As shown in Tables 3 and 4, as a pre-treatment beforeforming a protective film, an oxide semiconductor film was subjected toa nitrous oxide plasma treatment. Further, SiO_(x) was formed into afilm having a thickness of 100 nm by the plasma CVD (PECVD) method andSiN_(x) was formed on the SiO_(x) film in a thickness of 150 nm by theplasma CVD (PECVD) method to obtain a protective film. A contact holewas formed by dry etching, whereby a thin film transistor wasfabricated.

[Evaluation of Thin Film Transistor]

For the thus fabricated thin film transistor, a field effect mobility(p), an S value and a threshold voltage (Vth) were evaluated. Thesecharacteristic values were measured by using a semiconductor parameteranalyzer (4200SCS, manufactured by Keithley Instruments, Inc.) at roomtemperature in a light-shielding environment (in a shield box).

For the produced transistor, transfer characteristics were evaluatedwith the drain voltage (Vd) and the gate voltage (Vg) being 1V and −15to 20V, respectively. The results are shown in Tables 3 and 4. The fieldeffect mobility (μ) was calculated from the linear mobility, and definedas the maximum value of Vg-μ.

For the fabricated thin film transistor, a DC bias stress test wasconducted. Tables 3 and 4 show a change in TFT transfer characteristicsbefore and after application of a DC stress of Vg=15V and Vd=15V (stresstemperature: 80° C. or less) for 10000 seconds.

It was revealed that the thin film transistor in Examples 7 to 12suffered only a slight change in threshold voltage, i.e. it was hardlyaffected by a DC stress.

Comparative Examples 3 and 4

By using the 4-inch targets fabricated in Comparative Examples 1 and 2,oxide semiconductor thin films and thin film transistors were fabricatedand evaluated in the same manner as in Examples 7 to 12 in accordancewith the sputtering conditions, heating (annealing) conditions and apre-treatment for forming a protective film shown in Table 4. Theresults are shown in Table 4.

As shown in Table 4, in the devices in Comparative Examples 3 and 4, thethreshold voltage was negative. As a result of a stress test, in thethin film transistors of Comparative Examples 3 and 4, the thresholdvoltage varied by 1V or more, revealing that significant deteriorationin characteristics occurred.

Examples 13 to 15

By using the film-forming apparatus disclosed in JP-A-2005-290550 and inaccordance with the conditions shown in Table 5, AC sputtering wasconducted. Oxide sintered bodies and thin film transistors werefabricated in the same manner as in Examples 7 to 12, except that anamorphous film was formed under the following conditions and thesource/drain patterning was conducted by dry etching. The results areshown in Table 5.

As a result of the ICP-AES analysis of the oxide semiconductor thinfilm, it was confirmed that the atomic ratio of each element containedin the oxide thin film was the same as that of the sputtering target.

The AC sputtering was specifically conducted as follows by using theapparatus shown in FIG. 1.

6 targets 31 a to 31 f (each having a width of 200 mm, a length of 1700mm and a thickness of 10 mm) fabricated in Example 1, 3 or 4 were used.These targets 31 a to 31 f were arranged in parallel to the direction ofthe width of a substrate such that they remote from each other with aninterval of 2 mm. The width of the magnetic field forming means 40 a to40 f was 200 mm as in the case of targets 31 a to 31 f. From the gassupply system, Ar, H₂O and/or O₂ as the sputtering gas were respectivelyintroduced into the system.

The sputtering conditions for Example 13, for example, were as follows.Film-forming atmosphere: 0.5 Pa, AC source power density: 3 W/cm² (=10.2kW/3400 cm²), and frequency: 10 kHz.

In order to examine the film-forming speed, under the above-mentionedconditions, film formation was conducted for 10 seconds. The filmthickness of the resulting thin film was measured and found to be 13 nm.The film formation speed was as high as 78 nm/min, which is suited tomass production.

The glass substrate on which a thin film had been formed was put in anelectric furnace, and subjected to a heat treatment in the air at 300°C. for 60 minutes (in the air atmosphere). The thin film was cut into asize of 1 cm², and then subjected to the Hall measurement by the fourpoint probe method. As a result, the carrier concentration was 4.83×10¹⁷cm⁻³, indicating that the film became a sufficient semiconductor.

As a result of an XRD measurement, it was confirmed that the oxide thinfilm was amorphous immediately after the thin film deposition, and wasstill amorphous after allowing it to stand in the air at 300° C. for 60minutes.

TABLE 5 Example 13 Example 14 Example 15 Target composition In/(In +Sn + In/(In + Sn + In/(In + Sn + Zn + Al) = 0.50 Zn + Al) = 0.50 Zn +Al) = 0.50 Sn/(In + Sn + Sn/(In + Sn + Sn/(In + Sn + Zn + Al) = 0.15Zn + Al) = 0.10 Zn + Al) = 0.05 Zn/(In + Sn + Zn/(In + Sn + Zn/(In +Sn + Zn + Al) = 0.25 Zn + Al) = 0.30 Zn + Al) = 0.35 Al/(In + Sn +Al/(In + Sn + Al/(In + Sn + Zn + Al) = 0.10 Zn + Al) = 0.10 Zn + Al) =0.10 Sputtering Ultimate pressure (Pa) 5.0 × 10⁻⁵ 5.0 × 10⁻⁵ 5.0 × 10⁻⁵conditions Sputtering pressure (Pa) 0.5 0.5 0.5 [H₂O]/([H₂O] + [Ar] +[O₂]) (%) 0 1 1 [Ar]/([H₂O] + [Ar] + [O₂]) (%) 80 89 89 [O₂]/([H₂O] +[Ar] + [O₂]) (%) 20 10 10 Water partial pressure (Pa) 0 0.005 0.005Sputtering method AC AC AC AC power density (W/cm²) 3 5 5 AC frequency(Hz) 10k 20k 500k Film thickness (nm) 40 40 40 Substrate temperature (°C.) 80 80 80 Annealing Annealing temperature (° C.) 300 300 300Annealing time (min) 60 60 60 Atmosphere Air Air Air Hall Carrierconcentration (cm⁻³) 4.83E+17 6.34E+17 4.72E+17 measurement TFT Channelwidth/channel length (μm) 20/5 20/5 20/5 Source/drain Mo Mo MoSource/drain patterning Dry etching Dry etching Dry etching Protectivefilm SiOx/SiNx SiOx/SiNx SiOx/SiNx Mobility (cm²/Vs) 19.7 16.8 18.0Threshold voltage (V) 1.6 1.0 0.82 S value (V/dec) 0.11 0.12 0.16

INDUSTRIAL APPLICABILITY

The thin film transistor obtained by using the sputtering target of theinvention can be used for a display, in particular, for a large-areadisplay.

Although only some exemplary embodiments and/or examples of theinvention have been described in detail above, those skilled in the artwill readily appreciate that many modifications are possible in theexemplary embodiments and/or examples without materially departing fromthe novel teachings and advantages of the invention. Accordingly, allsuch modifications are intended to be included within the scope of theinvention.

The specification of the Japanese patent application to which thepresent application claims priority under the Paris Convention areincorporated herein by reference in their entirety.

1. A sputtering target comprising an oxide that comprises an indium (In) element, a tin (Sn) element, a zinc (Zn) element and an aluminum (Al) element, wherein the oxide comprises a homologous structure compound represented by InAlO₃(ZnO)_(m) (m is 0.1 to 10) and a bixbyite structure compound represented by In₂O₃.
 2. The sputtering target according to claim 1, wherein the homologous structure compound is one or more selected from homologous structure compounds represented by InAlZn₄O₇, InAlZn₃O₆, InAlZn₂O₅ and InAlZnO₄.
 3. The sputtering target according to claim 1, wherein the atomic ratio of In, Sn, Zn and Al satisfies the following formulas (1) to (4): 0.10≦In/(In+Sn+Zn+Al)≦0.75  (1) 0.01≦Sn/(In+Sn+Zn+Al)≦0.30  (2) 0.10≦Zn/(In+Sn+Zn+Al)≦0.70  (3) 0.01≦Al/(In+Sn+Zn+Al)≦0.40  (4) wherein in the formulas In, Sn, Zn and Al independently indicate an atomic ratio of each element in the sputtering target.
 4. The sputtering target according to claim 1 that has a relative density of 98% or more.
 5. The sputtering target according to claim 1 that has a bulk specific resistance of 10 mΩcm or less.
 6. The sputtering target according to claim 1 that does not comprise a spinel structure compound represented by Zn₂SnO₄.
 7. An oxide semiconductor thin film formed by a sputtering method with the use of the sputtering target according to claim
 1. 8. A method for producing an oxide semiconductor thin film, wherein the film is formed by a sputtering method with the use of the sputtering target according to claim 1 in an atmosphere of a mixed gas that comprises: one or more selected from water vapor, an oxygen gas and a nitrous oxide gas; and a rare gas.
 9. The method for producing an oxide semiconductor thin film according to claim 8, wherein the formation of the oxide semiconductor thin film is conducted in an atmosphere of a mixed gas that comprises a rare gas and at least water vapor.
 10. The method for producing an oxide semiconductor thin film according to claim 9, wherein the ratio of the water vapor contained in the mixed gas is 0.1% to 25% in terms of a partial pressure ratio.
 11. The method for producing the oxide semiconductor thin film according to claim 8 comprising: transporting substrates in sequence to positions opposing to 3 or more of the sputtering targets arranged in parallel with a prescribed interval in a vacuum chamber; applying a negative potential and a positive potential alternately from an AC power source to each of the targets; and causing plasma to be generated on the target by applying at least one output from the AC power source while switching the target to which a potential is applied among two or more targets that are divergently connected to this AC power source, thereby forming a film on a substrate surface.
 12. The method for producing an oxide semiconductor thin film according to claim 11, wherein the AC power density of the AC power source is 3 W/cm² or more and 20 W/cm² or less.
 13. The method for producing an oxide semiconductor thin film according to claim 11 or 12, wherein the frequency of the AC power source is 10 kHz to 1 MHz.
 14. A thin film transistor comprising, as a channel layer, the oxide semiconductor thin film formed by the method for producing an oxide semiconductor thin film according to claim
 8. 15. The thin film transistor according to claim 14 that has a field effect mobility of 10 cm²/Vs or more. 